1. Field of the Invention
The present invention relates to a longitudinal magnetic recording medium wherein information recording is conducted by magnetization in a longitudinal direction in the plane of the recording layer, which has magnetic properties remarkably improved.
2. Description of the Prior Art
In recent years, the application range of magnetic recording apparatus such as magnetic disc apparatus, flexible disc apparatus or magnetic tape apparatus has been remarkably expanded, and the importance of such magnetic recording apparatus has increased. Accordingly, for the magnetic recording media used in such apparatus, a remarkable improvement is being made in the recording density.
For such magnetic recording media, it is desired to attain a still higher recording density. For this purpose, the magnetic recording layer is required to have a high coercive force, and at the same time, the longitudinal magnetic recording medium wherein the magnetic recording is conducted in the circumferential direction of media, is required to have superior magnetic properties in the circumferential direction as compared with the properties in the radial direction of media.
As the magnetic layer for the longitudinal magnetic recording medium, a Co-Ni-Pt system and a Co-Ni, Co-Ni-Cr or Co-Cr system with a non-magnetic chromium primer layer are known. However, their coercive force is inadequate for high recording density media, and their magnetic properties in the recording direction are not good enough as compared with the properties in the traversed direction to the recording direction.
Further, in the summaries of reports presented at the 11th meeting of Japan Applied Magnetism Association, 18, 1987, it has been reported that the coercive force of a Co-Ni-Cr magnetic layer provided with a non-magnetic chromium primer layer, increases as the substrate temperature is raised. The reason for this is assumed to be the promotion of segregation of chromium to the crystal grain boundaries in the magnetic layer. However, this is only an assumption and is not confirmed. The segregation state of chromium changes depending upon the layer-forming conditions as described hereinafter. It is an important issue how the segregation state should be controlled to obtain desired magnetic properties of the longitudinal magnetic recording medium. Further, it is also an important question how to control the crystal orientation of the magnetic layer. However, in the above report, it has been reported that there is no change in the crystallizability. Furthermore, the necessity to raise the substrate temperature to a level of 290.degree. C. limits the practical applicability in view of the required flatness of the substrate and the magnetization of the substrate.
It has been reported that in a perpendicular magnetic recording medium comprising Co and Cr as the main components, the precipitation state of Cr in the crystal grains gives a substantial influence over the magnetic properties, for example, in Jpn. J. Appl. Phys. 24 (1985) L951, and IEEE Trans. Mag. MAG-23 (1987) 2061. Namely, it has been reported that by the segregation of Cr, ferromagnetic high Co regions will be formed in a plate-like shape periodically within the crystal grains in a direction vertical to the magnetic recording layer, whereby the magnetic properties in the direction vertical to the plane of the magnetic layer, such as the coercive force in the vertical direction and the vertical magnetic anisotropy constant, will be remarkably improved. Further, Japanese Unexamined Patent Publication No. 255813/1988 discloses that it is possible to improve the vertical magnetic properties by adjusting the average period of the segregation regions to a level of from 30 to 300 .ANG.. However, according to this mechanism, the (002) plane of cobalt crystals of the high cobalt concentration regions surrounded by high chromium concentration regions, is strongly oriented in the vertical direction by the plate-like segregation structure with the direction vertical to the plane of the magnetic layer being longitudinal, and the vertical magnetic anisotropy is created by the multiplication of the shape magnetic anisotropy by the plate-like structure.
Although such a chromium segregation structure may be optimum for vertical magnetic recording media, it is not suitable for longitudinal magnetic recording media, since the magnetization direction is different from the longitudinal magnetic recording media, and the magnetic properties in the longitudinal direction deteriorate.
Further, Japanese Examined Patent Publication No. 53770/1986 discloses that in a vertical magnetic recording medium comprising Co and Cr, or Co, Cr and Ni as the main components, the corrosion resistance can be improved by segregating Cr at the surface of crystals of the magnetic layer. However, this is also an invention for vertical magnetic properties, which are unsuitable for a longitudinal magnetic recording medium.
Further, in order to improve the magnetic properties in the recording direction, it has been attempted to impart a good magnetic anisotropy in the circumferential direction of a disc medium by forming a magnetic layer on a substrate having a texture in the circumferential direction. However, the effects are not remarkable within the range of practical surface roughness. In order to impart a remarkable magnetic anisotropy, a substrate having a texture in the circumferential direction is used, and a magnetic layer is formed by oblique incidence while rotating the substrate, as reported in J. Appl. Phys. 55 (1984) 15. However, this method is not suitable for mass production of the product.
Thus, a chromium segregation structure suitable for a longitudinal magnetic recording medium to obtain a high coercive force has not yet been known, and there has been no practical means to improve the magnetic properties in the direction of recording.